Brighton secured their Premier League safety as they deservedly beat Manchester United with a goal awarded by goalline technology.
Pascal Gross' header from Jose Izquierdo's cross was cleared by Marcos Rojo – but from just behind the line.
Albion – who had not won in their previous seven games – had the better chances and David de Gea did well to deny Glenn Murray and Izquierdo.
Marcus Rashford forced a good save from Mat Ryan in the second half.
Brighton players and their families celebrated on the pitch afterwards – it was their final home game of the season
The Seagulls were looking comfortable on 4 March when they beat Arsenal 2-1 to move into the top half of the Premier League. But seven games without a win in all competitions since then had seen some doubt creep in.
However, after moving on to 41 points and into 11th place, they cannot be caught by the bottom three – prompting a party atmosphere at the final whistle – and Chris Hughton can start getting ready for the club's second season in the top flight since 1983.
Anthony Knockaert and Izquierdo were fantastic on the two wings – likewise Gross in the centre – and caused United problems from start to finish.
Albion started brightly and Dale Stephens shot wide in the eighth minute. United keeper De Gea made two decent saves to stop Gross – and two fantastic diving saves to keep out Murray's 25-yard shot on the bounce and Izquierdo's rising hit.
The Seagulls fully merited their goal when Gross beat Ashley Young in the air to send Izquierdo's cross past De Gea.
The summer signing from Ingolstadt has been involved in 45% of Brighton's Premier League goals all season – seven goals and eight assists.
Brighton did not create many chances following that goal but showed good game management to see the game out.
Albion are now eight points above the relegation zone, but because Swansea and Southampton, who have three games left, play each other on Tuesday, it is impossible for both sides to get enough points to catch Brighton.
Manchester United boss Jose Mourinho says whether his side finish second in the league will determine whether the FA Cup finalists have had a good season overall.
They remain five points clear of third-placed Liverpool, although Tottenham – one point further back with a game in hand – can also catch them.
With Romelu Lukaku and Alexis Sanchez injured, Marcus Rashford got his chance in the centre-forward position but failed to take his opportunity.
United only had one touch in the Albion area in the first half, and their first effort on target did not come until the 49th minute when Paul Pogba's low shot was saved from 16 yards out.
They did create some chances after Gross' goal with Rashford and substitute Jesse Lingard forcing saves from outside the box and Anthony Martial striking two efforts over the bar from 20 yards.
The Red Devils did have one final chance but Lingard shot wide under pressure from Shane Duffy.
This is the first time United have ever lost away to three newly promoted clubs in a league season – they have been beaten at Huddersfield, Newcastle and now at the Amex Stadium, the 58th different ground they have visited in the Premier League.
Brighton boss Chris Hughton to BBC Sport: "Manchester United showed the quality they've got. Without us putting in the work we did, we wouldn't have got anything from the game.
"We've had three results – against Arsenal, Tottenham and Manchester United – where we've had to put in that kind of performance, and be good with the ball too. We deserved it tonight. Knockaert was outstanding, not just with the ball. Against teams like this you've got to work as hard without possession. It's good to see him in that kind of form.
"With Pascal Gross, what's been important for us is he hasn't been injured and he's always been available for us. We've probably been fortunate we've had a lot of players available in that area of the pitch."
Manchester United boss Jose Mourinho to BBC Sport: "Football was fair to the Brighton boys, they had a big target, they showed the point would be a big thing for them and gave everything for that.
"I couldn't persuade my players that second place was very important to us. I know we can do it, we only have two matches to get the four points we need. They beat us in the attitude.
"For 10 months I get asked 'why always Lukaku? Why always Lukaku, why always Lukaku? Why always this player? That guy doesn't have a chance to start, the other one is on the bench.' You know why now."